<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://unpkg.com/vue@next"></script>
</head>
<body>
    <div id="app">
        firstName:<input v-model="firstName"/><br>
        lastName:<input v-model="lastName"/><br>
        <h1>{{fullName}}</h1>
    </div>
    <script>
        Vue.createApp({
            data(){
                return{
                    firstName:"eric",
                    lastName:"lee",
                    fullName:"eric lee", 
                }
            },
            watch:{
                firstName(val){
                    this.fullName=val+" "+this.lastName;
                },
                lastName(val){
                    this.fullName=this.firstName+" "+val;
                }
            }
        }).mount("#app")
    </script>
</body>
</html>